Transmission and method of controlling clutch during ratio change

ABSTRACT

A transmission and control method are disclosed which ensure proper stroke pressure and minimize torque transients during a shift event. The transmission includes a clutch having a torque capacity based on a fluid pressure, a torque sensor adapted to measure a torque value that varies in relationship to the torque capacity, and a controller. The method includes varying the fluid pressure around a predetermined value, measuring a resulting torque difference with the torque sensor, and adjusting a clutch control parameter if the resulting torque difference is less than a threshold value.

TECHNICAL FIELD

The present disclosure relates to a system and method to control ratiochanges in an automatic vehicle transmission.

BACKGROUND

Known automatic transmissions for automotive vehicles include step ratiocontrols for effecting speed ratio changes in response to changingdriving conditions. The term “speed ratio”, for purposes of thisdescription, is defined as transmission input shaft speed divided bytransmission output shaft speed.

An upshift occurs when the driving conditions require a ratio changefrom a lower numbered ratio (high speed ratio) to a higher number ratio(low speed ratio) in the transmission gearing. Similarly, a downshiftoccurs when the driving conditions require a ratio change from a highernumbered ratio (low speed ratio) to a lower number ratio (high speedratio). The gearing can include, for example, either a planetary typegear system or a lay shaft type gear system. An automatic gear ratioshift is achieved by friction torque establishing devices, such asmultiple disk clutches and multiple disk brakes. The friction torqueestablishing devices include friction elements, such as multiple plateclutches and band brakes, which can be actuated hydraulically.

A step-ratio automatic transmission uses multiple friction elements forautomatic gear ratio shifting. A ratio change occurs in a synchronousclutch-to-clutch shift as one friction element, which may be referred toas the oncoming clutch (OCC), is engaged and a second friction element,which may be referred to as the off-going clutch (OGC), is disengaged.Failure to properly coordinate the engagement of the OCC with thedisengagement of the OGC can be perceived by the vehicle occupants as anunpleasant shift event. More particularly, early engagement of the OCCrelative to the release of the OGC can result in a phenomenon calledtie-up. On the other hand, if the OCC is engaged too late relative tothe release of the OGC, an engine flare can occur.

SUMMARY

In one embodiment, a method for controlling a transmission is provided.The method ensures proper clutch stroke and minimizes torque transients.During a downshift, a clutch pressure is set for an oncoming clutch at apredetermined stroke pressure. Then the clutch pressure is varied fromthe predetermined stroke pressure. A resulting torque difference ismeasured along a torque transmitting element with a torque sensor whilethe clutch pressure is varied. A clutch control parameter is adjusted ifthe resulting torque difference is less than a threshold value.

In another embodiment, the torque transmitting element can be, forexample, an input shaft or an output shaft.

In yet another embodiment, varying the clutch pressure can involvepulsing the clutch pressure above the predetermined stroke pressure,pulsing the clutch pressure below the predetermined pressure, graduallyincreasing the clutch pressure in a ramp profile, or other means.

In some embodiments, the method can include setting the clutch pressureat a boost pressure higher than the predetermined stroke pressure for aboost duration before setting the clutch pressure at the predeterminedstroke pressure.

In still another embodiment, the clutch control parameter to be adjustedcan be, for example, the predetermined stroke pressure, the boostpressure, or the boost duration.

In one other embodiment, a method for controlling a transmission isprovided. The method includes varying a clutch pressure around apredetermined value in advance of a torque phase of a shift event. Atorque change is measured in a transmission element as the clutchpressure is varied. A clutch control parameter is adjusted in responseto the measured torque change.

In another embodiment, the value can be increased if the change inmeasured torque is below a first threshold.

In another embodiment, the value can be decreased if the change inmeasured torque is above a second threshold.

In another embodiment, the shift event can be a downshift and the clutchcan be the oncoming clutch for the downshift.

In on other embodiment, a transmission is provided. The transmissionincludes a clutch having a torque capacity based on a fluid pressure anda torque sensor adapted to measure a torque value that varies inrelationship to the torque capacity. A transmission controller isconfigured to vary the fluid pressure from a predetermined value inadvance of a torque phase of a shift event and adjust the predeterminedvalue in response to a change in the measured torque value.

The above advantages and other advantages and features will be readilyapparent from the following detailed description of the preferredembodiments when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a transmission;

FIG. 2 is a schematic diagram of a transmission clutch or brake;

FIG. 3 is a graph illustrating a downshift under idealized clutchpressure control;

FIG. 4 is a graph illustrating a downshift under open loop clutchpressure control in which the oncoming clutch pressure is set too high;

FIG. 5 is a graph illustrating a downshift under open loop clutchpressure control in which the oncoming clutch pressure is set too low;

FIG. 6 is a flow chart illustrating a first embodiments of a closed looppressure control algorithm;

FIG. 7 is a graph illustrating a downshift under the closed loop clutchpressure control system of FIG. 6 in which the initial oncoming clutchpressure is set too high;

FIG. 8 is a graph illustrating a downshift under the closed loop clutchpressure control system of FIG. 6 in which the initial oncoming clutchpressure is set too low;

FIG. 9 is a flow chart illustrating a second embodiments of a closedloop pressure control algorithm; and

FIG. 10 is a graph illustrating a downshift under the closed loop clutchpressure control system of FIG. 9.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely examples of the invention that can be embodied in various andalternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

FIG. 1 illustrates a six speed planetary transmission 18 with threeplanetary gear sets 20, 22, and 24. Each planetary gear set includes asun gear, a ring gear, a planet carrier, and a collection of planetgears supported for rotation about the planet carrier and meshing withboth the sun gear and the ring gear. The carrier of gear set 20 isfixedly connected to the ring gear of gear set 22, the carrier of gearset 22 is fixedly connected to the ring gear of gear set 24, and thecarrier of gear set 24 is fixedly connected to the ring gear of gear set20. Input shaft 26 is fixedly connected to the sun gear of gear set 22and output shaft 28 is fixedly connected to the carrier of gear set 24.

Various power flow paths between input shaft 26 and output shaft 28 areestablished by the selective engagement of clutches and brakes. Brakes30, 32, and 34 selectively hold the sun gear of gear set 20, the carrierof gear set 20, and the sun gear of gear set 24, respectively, againstrotation. Clutches 36 and 38 selectively connect the sun gear of gearset 20 and the carrier of gear set 20, respectively, to input shaft 26.Table 1 indicates which clutches and brakes are engaged in order toestablish each of the six forward and one reverse transmission ratios.Torque sensor 40 senses the torque transmitted to the output shaft andelectrically communicates that information to controller 42. Thecontroller 42 can, for example, be part of a vehicle system controlmodule or transmission control module or can be a stand-alonecontroller.

TABLE 1 Brake 30 Brake 32 Brake 34 Clutch 36 Clutch 38 Reverse X X 1st XX 2nd X X 3rd X X 4th X X 5th X X 6th X X

While an automatic transmission according to an embodiment of thedisclosure can be a planetary type as shown in FIG. 1, it is alsocontemplated that the transmission can be a lay shaft type transmission.Similarly, a speed ratio change can be achieved by the friction elementsas described above, or the friction elements can be plate clutches orband brakes.

FIG. 2 illustrates a representative cross section of a clutch, such asclutches 36 and 38 and brakes 30, 32, and 34 in FIG. 1. A set offriction plates 44 is splined to a clutch hub 46. The friction plates 44are interspersed with a set of separator plates 48 that is splined to aclutch cylinder 50. In the disengaged state as shown here in FIG. 2,there is space between the friction plates 44 and the separator plates48 such that the hub 46 and the cylinder 50 are free to rotate atdifferent speeds with respect to each other.

To engage the clutch, pressurized fluid is forced into the cylinder 50.The pressure is supplied by a pump 52. The controller 42 regulates thehydraulic pressure indirectly by setting an electrical current in asolenoid 54 which controls the position of a valve 56. The pressurizedfluid travels through a hydraulic passageway 58 to the clutch cylinder50. The pressurized fluid forces the piston 60 to slide within thecylinder 50 and squeeze the friction plates 44 and separator plates 48together. Friction between the friction plates 44 and the separatorplates 48 resists relative rotation of hub 46 and cylinder 50. When thefluid pressure is removed, a return spring 62 forces the piston 60 toslide in the opposite direction returning the clutch to the disengagedstate.

The torque capacity of the clutch depends upon the fluid pressure butthe relationship is complicated by several factors. First, there is atime delay between when fluid starts flowing to the cylinder 50 and whenthe piston 60 has moved far enough to start squeezing the frictionplates 44 and separator plates 48 together. The torque capacity of theclutch is nearly zero during this period before the piston 60 is fullystroked. When the piston 60 has moved such that it can apply force tothe plates 44, 48, the piston and clutch are said to be stroked.Secondly, some amount of pressure, called the stroke pressure, isrequired to overcome the force of the return spring 62 even after thepiston 60 is stroked.

Once the piston 60 is stroked, the clutch torque capacity isproportional to the fluid pressure minus the stroke pressure. However, avariety of unpredictable noise factors influence the relationshipbetween the solenoid 54 current as commanded by the controller 42 andthe torque capacity so that the commanded torque capacity may not beaccurately achieved. For example, variations in the coefficient offriction, frictional forces between the piston 60 and the cylinder 50,and pressure variations in the passageway 58, may cause the actualtorque capacity to be either higher or lower than commanded.

These noise factors can make it difficult to achieve a smooth shiftbehavior without torque transient conditions that may be perceptible toa driver. A downshift from one speed ratio to another requires thecoordinated application of one clutch and release of another. Forexample, to shift from sixth gear to fifth gear, brake 30 (the OGC) isreleased while clutch 38 (the OCC) is applied, as described in Table 1.As discussed above, noise factors make it more difficult to achieve asmooth shift behavior using only open loop control strategies. Thedisturbances associated with pressure control inaccuracy are bestunderstood in relation to the intended behavior which is illustrated inFIG. 3. As discussed below, actual control strategies do not repeatablyachieve this behavior.

FIG. 3 illustrates how a downshift process would ideally be executed ifthere were no noise factors and the controller could command preciselythe right amount of torque capacity. The holding pressure for the OGCwould be set to the pressure at which the torque capacity of the OGCequals the torque carried by the OGC in the initial gear. To initiatethe shift, the controller would reduces the pressure to the OGC to alevel slightly below the holding pressure as shown at 102, marking thebeginning of the inertia phase.

During the inertia phase, the input speed would increase to the correctmultiple of the output speed for the destination ratio, as shown at 104.The output torque would drop slightly, as shown at 106, because some ofthe input power would be consumed to overcome the inertia of elementsconnected to the input. During the inertia phase, the OCC would bestroked in preparation for the torque transfer phase. The commandedpressure to the OCC would be elevated to a high pressure, P_(boost), fora short interval, t_(boost), to rapidly fill the cylinder with fluid andmove the piston to the stroke position, as shown at 108. Then, thecommanded pressure would be maintained at a pressure near the strokepressure. In FIG. 3, the actual pressure is shown equal to the strokepressure at 110, which would keep the piston stroked but not apply anytorque.

Once the input speed reaches the correct multiple of the output speed at112, the torque transfer phase begins. During the torque transfer phase,the commanded pressure to the OGC would be gradually reduced 114 whilethe commanded pressure to the OCC is gradually increased 116. Ideally,the torque capacity of the two clutches would be coordinated such thatthe input speed remains constant 118 and the output torque graduallyincreases 120. The torque transfer phase is complete when the OCCpressure is above its holding pressure 122 and the OGC pressure is belowits stroke pressure 124. The commanded pressure of the OCC would then befurther increased to provide some margin over the holding pressure asshown at 126.

While FIG. 3 illustrates an ideal system without noise factors, theactual pressure will generally only approximate the stroke pressure. Inthe absence of a feedback signal, it is difficult to determine if thecommanded pressure has being achieved.

FIGS. 4-5 illustrate the potential problems associated with the noisefactors and subsequent pressure control errors in an open loop controlstrategy. FIG. 4 illustrates an effect of accidentally commanding an OCCpressure above the stroke pressure during the inertia phase 128. Oncethe OCC is stroked, the torque capacity increases to a positive value130. Since the speed ratio at this point is below the speed ratio of thedestination gear, torque capacity of the OCC produces a drop in theoutput torque 132. The vehicle occupants perceive this fluctuation inoutput torque as a rough and jerky shift event.

FIG. 5 illustrates an effect of accidentally commanding an OCC pressurebelow the stroke pressure 134. In this circumstance, the OCC is notfully stroked by the beginning of the torque transfer phase. As thecommanded pressure of the OCC is increased in the torque transfer phase,there is a delay before the OCC torque capacity begins to increase 136.During this delay period, the input speed continues to increase abovethe speed ratio of the destination gear as shown at 138. This is calledan engine flare. Eventually, the OCC torque capacity increases enough tobring the input speed back to the desired level 140. The output torquechanges suddenly 142 when the input speed returns to the destinationgear speed ratio which occupants perceive as a rough and jerky shiftevent.

FIG. 6 illustrates a flow chart of a control system for a transmissionusing closed loop control during a ratio shift. As those of ordinaryskill in the art will understand, the functions represented by theflowchart blocks can be performed by software and/or hardware. Also, thefunctions can be performed in an order or sequence other than thatillustrated in FIG. 6. Similarly, one or more of the steps or functionscan be repeatedly performed although not explicitly illustrated.Likewise, one or more of the representative steps of functionsillustrated can be omitted in some applications. In one embodiment, thefunctions illustrated are primarily implemented by softwareinstructions, code, or control logic stored in a computer-readablestorage medium and if executed by a microprocessor based computer orcontroller such as the controller 50.

FIG. 6 is a flow chart for one embodiment of the present disclosure forusing a torque sensor for detecting improper stroke and using closedloop control during a ratio shift. Initially when a ratio shift isrequested, the controller raises the OCC pressure to a boost pressureP_(boost) for a boost time t_(boost) in order to quickly move the pistonto a substantially stroked position, as represented by blocks 60 and 62.The boost pressure P_(boost) is a clutch control parameter significantlyabove the stroke pressure P_(stroke). For example, the boost pressurecan be the maximum available pressure based on limits of the solenoid.The boost time t_(boost) is a clutch control parameter calculated to belong enough to substantially stroke the clutch and short enough that theclutch does not prematurely transmit torque.

Then, the controller commands the OCC to an estimated stroke pressureP_(stroke) _(—) _(est) and waits for a period t_(test) calculated to belong enough for the piston to reach an equilibrium position asrepresented by blocks 64 and 66. Both P_(stroke) _(—) _(est) andt_(test) are clutch control parameters. Initial values for all clutchcontrol parameters can be established experimentally based on vehicletesting and can be adjusted adaptively during vehicle operation. In thisillustrative example, P_(stroke) _(—) _(est) is adjusted adaptively.

At 68, the controller records a reference reading τ_(ref) from a torquesensor 40. The torque sensor can measure the torque on the output shaftas shown in FIG. 1, the input shaft, or any other element that transmitstorque in the destination gear. At 70, the controller commands apressure variation P_(test) above or below the estimated stroke pressureP_(stroke) _(—) _(est). The incremental pressure P_(test) is calculatedto be enough of a pressure variation to generate a change in the torquemeasured by the torque sensor 40 if the clutch is fully stroked.However, the pressure variation can be small enough that the change intorque would not be objectionable or even noticeable to the vehicleoccupants.

At 72, the controller records a second reading τ_(test) from the torquesensor 40. At 74, the controller compares the two torque readings,τ_(ref) and τ_(test), to determine if the difference between τ_(ref) andτ_(test) differ by more than a threshold amount τ_(threshold). Thethreshold amount τ_(threshold) is calculated to be large enough thatshort term variations due to noise factors are not erroneouslyattributed to the change in commanded pressure. If the two pressures,τ_(ref) and τ_(test), differ by less than the threshold amountτ_(threshold), this is indicative that the piston was not fully stroked.If the piston is not fully stroked, then the estimated stroke pressureis increased as represented by block 76. On the other hand, if the twopressures, τ_(ref) and τ_(test), differ by more than the thresholdamount τ_(threshold), this is indicative that the piston was fullystroked. If the piston is fully stroked, then the estimated strokepressure is decreased, as represented by block 78. At 80, the controllercommands the revised estimated stroke pressure.

Finally, if there is time remaining before the end of the inertia phase,another adjustment is performed. Otherwise, the process ends and therevised estimated stroke pressure is utilized in future shift eventsinvolving that OCC.

FIG. 7 illustrates the results of utilizing the control strategy of FIG.6 when the initial estimated stroke pressure is higher than a requiredstroke pressure as shown at 144. When the estimated stroke pressure istoo high, the clutch is fully stroked and has positive torque capacity146. When the clutch is fully stroked, the upward 148 and downward 150perturbations in commanded pressure produce measurable changes in torqueas shown at 152 and 154 which are detectable by the torque sensor. Forexample, the upward perturbation 148 results in the measured torquereading τ_(test1) 156. The torque perturbation is compared to areference torque value τ_(ref1) 158. If the difference between themeasured torque readings, τ_(test1) and τ_(ref1), is greater than athreshold amount τ_(threshold), then the estimated stroke pressure isdecreased. The controller commands this decreased stroke pressure 160.

Prior to a second perturbation 150, a revised reference torque valueτ_(ref2) 162 is measured. Following the perturbation, a second torquereading τ_(test2) 164 is measured. Even though the new commandedpressure is below the required stroke pressure, the torque differencestill exceeds the threshold, resulting in another downward adjustment.The commanded pressure is set to the new adjusted value as show at 166.Please note, the perturbations in pressure and torque may be exaggeratedfor illustrative purposes.

FIG. 8 illustrates the results when the initial estimate of strokepressure is below the actual stroke pressure as shown at 168. When theestimated stroke pressure is too low, the clutch is not fully strokedand has zero torque capacity 170. In this unstroked condition,perturbations in commanded pressure do not produce a measurable change.For example, as illustrated, upward pressure pulse 172 and downwardpressure pulse 174 do not affect output torque 176. Consequently, thenthe estimated stroke pressure can be increased after each perturbation.The controller commands this increased stroke pressure as shown at 178and 180.

FIG. 9 is a flow chart for another embodiment of the present disclosurewhere the initial estimate of the stroke pressure is intentionally setslightly below the required stroke pressure and gradually increaseduntil a measurable change is detected. Blocks 60 through 68 areidentical to the previously described embodiment except that the initialestimate is decreased from the previous value at block 86. Blocks 88,90, 92, and 94 form a loop in which the estimated stroke pressure andthe commanded pressure is gradually increased until the torque sensorindicates a change in measured torque. The increment added to P_(stroke)_(—) _(est) in each iteration can be small compared to the incrementused at blocks 76 and 78 of FIG. 6 or block 86 of FIG. 9.

FIG. 10 illustrates the results of utilizing the control strategydescribed in FIG. 9. After the boost phase, the clutch pressure is setto a value below the stroke pressure at 182. Because the clutch is notfully stroked, the clutch torque capacity is zero 184. The referencetorque value τ_(ref) 186 is measured. Then, the commanded pressure isgradually increased, as shown at 188. Once the commanded pressurereaches the stroke pressure, the clutch torque capacity will begin toincrease above zero, as shown at 190, and the output torque will beginto decrease, as shown at 192. In each iteration, a new test torqueτ_(test) 194 is measured until the difference between the measuredtorque readings, τ_(test) and τ_(ref), is greater than a thresholdamount τ_(threshold) 196.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method for controlling a transmission, themethod comprising: setting a clutch pressure for an oncoming clutch at apredetermined stroke pressure during a current downshift event; varyingthe clutch pressure from the predetermined stroke pressure during aninertia phase of the current downshift event; measuring a resultingtorque difference along a torque transmitting element with a torquesensor while the clutch pressure is varied; and adjusting a clutchcontrol parameter if the resulting torque difference is less than athreshold value, wherein the clutch control parameter is adjusted duringone of the current downshift event and a future shift event, to ensureproper clutch stroke and minimize torque transients during a torquephase.
 2. The method of claim 1 wherein measuring the resulting torquedifference comprises measuring the torque transmitted by at least one ofan input shaft and an output shaft with a torque sensor.
 3. The methodof claim 1 wherein varying the clutch pressure comprises pulsing theclutch pressure above the predetermined stroke pressure.
 4. The methodof claim 1 wherein varying the clutch pressure comprises pulsing theclutch pressure below the predetermined stroke pressure.
 5. The methodof claim 1 wherein varying the clutch pressure comprises graduallyincreasing the clutch pressure in a ramp profile.
 6. The method of claim1 further comprising setting the clutch pressure at a boost pressure fora boost duration before setting the clutch pressure at the predeterminedstroke pressure, the boost pressure being higher than the strokepressure.
 7. The method of claim 6 wherein the clutch control parametercomprises at least one of the predetermined stroke pressure, the boostpressure, and the boost duration.
 8. A method for controlling atransmission, the method comprising: varying a clutch pressure around apredetermined value in advance of a torque phase of a shift event;measuring a torque change in a transmission element as the clutchpressure is varied; and adjusting a clutch control parameter in responseto the measured torque change.
 9. The method of claim 8 wherein thevalue is increased if the change in measured torque is below a firstthreshold.
 10. The method of claim 8 wherein the value is decreased ifthe change in measured torque is above a second threshold.
 11. Themethod of claim 8 wherein the shift event is a downshift and the clutchis the oncoming clutch for the downshift.
 12. The method of claim 8wherein the clutch pressure is increased above the value by a testpressure sufficient to cause a measurable increase in the torque in thetransmission element if the clutch is fully stroked.
 13. The method ofclaim 8 wherein the clutch pressure is decreased below the value by atest pressure sufficient to cause a measurable decrease in the torque inthe transmission element if the clutch is fully stroked.
 14. The methodof claim 8 wherein the commanded pressure is gradually increased untilthe measured torque in the transmission element changes.
 15. The methodof claim 8 wherein the transmission element is one of an input shaft andan output shaft.
 16. The method of claim 8 wherein the value is adjustedprior to the start of the torque phase.
 17. The method of claim 8wherein the value is adjusted for future shift events.
 18. Atransmission comprising: a clutch having a torque capacity based on afluid pressure; a torque sensor adapted to measure a torque value in ashaft wherein the torque value varies in relationship to the torquecapacity; and a controller configured to: vary the fluid pressure from apredetermined value in advance of a torque phase of a shift event; andadjust the predetermined value in response to a change in the measuredtorque value.
 19. The transmission of claim 18 wherein the shaft is oneof an input and an output.
 20. The transmission of claim 18 wherein theshift event is a downshift and the clutch is the oncoming clutch for thedownshift.